Recently, optical waveguide type optical modulators 140 as shown in FIG. 9 have become very common in optical fiber communication systems. In this optical waveguide type optical modulator 140, optical waveguides 142 are formed on a substrate 141 which has an electro-optic effect, and a guided lightwave which travels in he optical waveguides 142 is controlled by a traveling-wave type signal electrode 143a and grounded electrodes 143b. In the optical waveguide type optical modulator 140 of this example, normally, a buffer layer 144 made of an insulating material such as silicon oxide is formed on the substrate 141 to prevent absorption of the guided lightwave which travels in the optical waveguide 142 by the electrodes 143. Furthermore, a signal field adjustment region 145, which has wider width than that of the traveling-wave type signal electrode 143a, is formed between the traveling-wave type signal electrode 143a and the buffer layer 144, allowing the effective refractive index of the microwaves which propagate the electrodes 143 to be adjusted.
Such an optical waveguide type optical modulator 140 can be manufactured using the following method.
At first, after the optical waveguides 142 are formed by thermal diffusion on the surface of the substrate 141 which is made of a ferroelectric substance, the buffer layer 144 is formed on the substrate 141 using methods such as vacuum deposition or sputtering. Subsequently, the signal field adjustment region 145 is formed at a predetermined position on this buffer layer 144. Then, the traveling-wave type signal electrode 143a is formed at a predetermined position on the signal field adjustment region 145, and the ground electrodes 143b are formed at predetermined positions on the buffer layer 144, at different positions from the signal field adjustment region 145.
In such a manufacturing process, the signal field adjustment region 145 is normally formed using a lift-off method or an etching method.
In a lift-off method, at first, a photoresist is applied to the buffer layer 144. Then after exposing a desired pattern onto the photoresist using a photomask, this resist pattern is developed to make the masked portions, excluding the predetermined position where the signal field adjustment region 145 is to be formed. Subsequently, a film of the metal or semiconductor or the like which forms the signal field adjustment region 145 is deposited thereon, and by removing the resist using a resist removal agent, the film on the resist is removed at the same time, thereby forming the signal field adjustment region 145 at a predetermined position on the buffer layer 144.
An example of an etching method is a wet etching method employing a liquid etchant. In order to form the signal field adjustment region 145 using a wet etching method, firstly, a film of the metal or semiconductor or the like which forms the signal field adjustment region 145 is deposited on the buffer layer 144, and a photoresist is applied thereon. After exposing a desired pattern onto the photoresist using a photomask, this resist pattern is developed, masking the predetermined position where the signal field adjustment region 145 is to be formed. Then, the exposed portions of the film are removed using an etchant of mixed acid or the like. The remaining photoresist is then removed using a resist removal agent, thereby forming the signal field adjustment region 145 at a predetermined position on the buffer layer 144.
However, when the signal field adjustment region is formed on the buffer layer using a lift-off method, because the photoresist is applied directly to the buffer layer and hardened, the principal components and the diluent which constitute the photoresist may penetrate the buffer layer, causing contamination of the buffer layer. In particular, if the buffer layer is silicon oxide, the surface of the buffer layer is sometimes treated with vapor of an amine-based compound in order to improve the adhesive strength of the photoresist to the silicon oxide, and in such a case, the buffer layer can also be contaminated by the amine-based compound.
Furthermore, when the resist pattern is developed, or when a metal or semiconductor is deposited to form the signal field adjustment region, solutions such as resist developer or special chemical agents are used in these processes. Consequently, there is a danger that the penetration of the resist components into the buffer layer may be accelerated because components from the resist can dissolve in these solutions. Moreover, these solutions may also be a source of contamination themselves.
Furthermore, when the signal field adjustment region is formed using a wet etching method, there is a danger that the buffer layer is contaminated by contacting the etchant or the resist removal agent.
If the buffer layer is contaminated, DC drift in the optical waveguide type optical modulator is promoted by the presence of ions derived from the contaminants, thereby reducing the long-term reliability of the optical waveguide type optical modulator. Moreover, in the case that degree of contamination being high, the insulating properties of the buffer layer deteriorate greatly, allowing a portion of, or most of the electric field applied from the electrodes to be leaked through the buffer layer, which means that an electric field cannot be efficiently generated in the optical waveguide, and even though a signal field adjustment region is provided, the effect of the region may be inadequate.
Such contamination of the buffer layer has a great influence on the performance of the optical waveguide type optical modulator. In order to suppress this contamination, it is necessary to control the density and the microstructure or the like of the buffer layer itself, and also precisely control the conditions for the lift-off process and the etching process, but it is extremely difficult to control these conditions.
On the other hand, FIG. 10 shows a cross-sectional view of another example of a conventional optical waveguide type optical modulator. This example is similar to that shown in FIG. 9, but differs in terms of structure.
This optical waveguide type optical modulator uses a ferroelectric substrate made of lithium niobate (LiNbO3), which is the most common and practical material for optical waveguide type optical modulators using ferroelectric substrates.
In FIG. 10, reference numeral 210 indicates a Z-cut ferroelectric substrate made of lithium niobate. The axis inducing the electro-optic effect of this ferroelectric substrate 210 is in the direction of the Z axis, which is the main optical axis (the crystallographic c axis), and as shown in FIG. 10, is aligned with a direction orthogonal to the surface where the optical waveguides 202 are formed (termed as the “main surface” in this specification) in the ferroelectric substrate 210.
The optical waveguides 202, fabricated by thermal diffusion of Ti, are formed near the main surface of the ferroelectric substrate 210, and a buffer layer 203 made of SiO2 is formed thereon. In addition, electrodes 204 made of Au are formed on the buffer layer 203 so as to be parallel to the optical waveguides 202. A transition metal layer 205 consisting of a transition metal such as Ti, Cr, Ni or the like is provided between the electrode 204 and the buffer layer 203.
Such an optical waveguide type optical modulator can be manufactured using a method in which, firstly, the optical waveguides 202 are formed on the main surface of the ferroelectric substrate 210 by means of thermal diffusion, and then the buffer layer 203 is formed on the side of the ferroelectric substrate 210 on which the optical waveguides 202 are formed, using a method such as vacuum deposition or sputtering. Then, a transition metal film and an Au film are formed sequentially on the entire upper surface of the buffer layer 203 by vacuum deposition. A thick Au layer accumulated on this Au film by an electroplating process, only within an electrode forming region, where the electrodes 204 are to be formed, thereby forming the electrodes 204. Subsequently, the Au film and the transition metal film remaining between the electrodes 204 is removed by chemical etching, to obtain a transition metal layer 205.
However, because the buffer layer 203 of such an optical waveguide type optical modulator is exposed between the electrodes 204, the modulator has a disadvantage that an exposed surface 203a of the buffer layer 203 and the inside of the buffer layer 203 are easily contaminated by contaminants such as K, Ti and Cr.
In particular, when vacuum deposition method is used to form the buffer layer 203 with a low density so as to control the characteristics of the optical waveguide type optical modulator, a problem that contaminants can easily penetrate the buffer layer 203 via the exposed portions may be occurred.
If the surface 203a of the buffer layer 203 of the optical waveguide type optical modulator and the inside of the buffer layer 203 are contaminated, DC drift may occur. DC drift refers to a phenomenon that the presence of alkali ions such as K or Na and mobile ions like proton causes the electric current leakage applied to the electrode 204 through the buffer layer 203, causing that the desired voltage (bias) cannot be applied, which has a negative effect on the characteristics of the optical waveguide type optical modulator.
In addition, if the contaminants in the buffer layer 203 reach to the interface between the ferroelectric substrate 210 and the buffer layer 203 as a result of thermal treatment performed during the mounting process of modulator chip or the like, the chemical bonds of the SiO2 buffer layer 203 can be broken by the contaminants, reducing the bonds binding the ferroelectric substrate 210 comprising lithium niobate and the buffer layer 203. As a result, it is expected that the bonding strength between the ferroelectric substrate 210 and the buffer layer 203 is weakened remarkably.